In order to save power and increase the life span of batteries, the low power consumption design is highly demanded in integrated circuit chips for mobile devices. In the low power consumption design, turning off the power supply to idle circuit modules of the integrated circuit chips is one of the main ways to achieve power savings.
A power switch is usually designed in a chip to control the on/off state of a circuit module. When the circuit module is in an idle state, the power switch turns off the power supply to the circuit module to save power; while when the circuit module needs to be re-activated, the power switch turns on the power supply to the circuit module.
FIG. 1 is a circuit diagram of a power switch in a conventional low power consumption chip. As shown in the figure, the circuit module includes multiple PMOS transistors each receives an enable signal EN output from a power control circuit inside the chip to be switched on or switched off. Wherein, VDD indicates an external power supply, VDDC indicates a module power supply for the circuit module. The enable signal EN controls the connection between the external power supply VDD and the module power supply VDDC.
Here we use the PMOS to implement the power switch because a PMOS usually has a lower leakage current. However, NMOS can be used to implement the power switch too. The advantage of using NMOS is smaller area and faster switching speed.
FIG. 2 shows two layout structures of the power switch in the integrated circuit chip. As shown in the figure, the power switch can be arranged in the chip in the following two ways:
{circle around (1)} in columns crossing the circuit module
{circle around (2)} surrounding the periphery of the circuit module
Although such layout structures can effectively reduce the power consumption of the chip, it also bring some adverse effects. Specifically, great amounts of power switches are required to ensure sufficient power supply to the circuit module and reduced the on-resistance. However, the PMOS transistors also bring huge capacitive loads at the same time, which will cause a large transient current (inrush current) when charging the capacitive loads all at once.
It is well known to those skilled in the art that inrush current, also named surge, generates at the instant of turning on the power supply to the circuit module. In other words, a current instantly reaches its peak beyond a stable value. Such huge transient currents will negatively affect the reliability of the chip. If the power supply to the circuit module is not designed properly, the large current may cause electro migration and lead to an open circuit or a short circuit, resulting in abnormality or failure of the chip function. Furthermore, excessive transient currents may also burn the chip, and even cause a fire, or other equipment damage or personnel injuries.